Metal surface protection is important for a variety of applications including aircraft structural components, heat exchangers and electrical system housings. A number of coating approaches have been taken to protect metal surfaces. Chromate conversion coatings are sometimes used to replace native oxide films on metal surfaces because they possess desirable and predictable properties. For example, chromate conversion coatings offer active corrosion protection and promote adhesion of other coatings to aluminum alloys. However, the presence of hexavalent chromium, a carcinogen, in these coatings discourages their continued use.
One alternative to conversion coatings containing hexavalent chromium is trivalent chromium pretreatment (TCP). One such example has been developed by the U.S. Navy and is described in U.S. Pat. No. 6,375,726. This TCP process has seen use in automotive and architectural applications. However, the use of TCP coatings in aerospace applications is problematic due to base alloy properties and process sensitivities that yield inconsistent and short-duration passivity of treated metal surfaces. In conventional TCP processes, a metal substrate is dipped into a TCP solution for a specified length of time (generally 5 minutes or more). The chemical reactions in the TCP process are driven by the electrochemical potential of the metal substrate. For alloy systems, microscopic variations in the substrate's electrochemical potential exist due to micro scale intermetallic particles (precipitates that exist on the alloy surface). As a result, the conventional TCP process is difficult to control and unpredictable and does not produce a robust coating. TCP coating failures for alloys have been attributed to nonuniformity in the chemical composition across the intermetallic particles (IMs), which is believed to be due to diffusional mass transportation limitations of the chromium coating formed on the intermetallic particles.